Thermal insulating systems have been the subject of extensive developmental efforts, particularly insulating systems for aerospace application. Such systems include the use of high temperature thermal insulation such as that sold by Manville under their trademark Min-K. Min-K thermal insulation is available in a fabric formed from fibrous media and very fine heat-resistant particulate matter and creates a physically strong, microporous insulation with extremely low thermal conductivity (e.g., 0.20-0.26 BTU/in/FT.sup.2 -Hr-F.degree.) and low thermal diffusivity that is effective at high temperature and high altitudes.
At the other temperature extreme are the thermal insulative systems commonly used in vacuum-insulated cryogenic dewars and sometimes referred to as "super insulation" or "SI". Such systems can comprise multiple layers of a metallized film, such as metallized mylar, which has been provided with a distribution of protrusions to prevent face-to-face contact when the film is wrapped around the inner container of a vacuum insulated dewar, provided with a distribution of openings to permit evacuation of the intervening space between the inner dewar and outer shield of a vacuum insulated dewar, and provided with a metallized surface to reduce heat transfer by radiation between the inner container and outer shield of a vacuum-insulated dewar.
Static systems for providing thermal insulation at normal atmospheric conditions commonly include the use of foam-like insulation formed from catalyzed liquid including blowing agents that are released by reaction in response to the catalyst to form cellular solids with a fine cell structure. Such insulating systems include polyurethane and isocyanate foam systems, and styrene-based foam systems such as those sold by Dow Chemical Co. under their trademark STYROFOAM.
Other systems have also been suggested for normal atmospheric conditions. For example, U.S. Pat. No. 4,367,788 discloses the use of a porous material, such as perlite, mixed with a suitable phase-change compound, such as sodium sulfate decahydrate (Glauber's salt), having a high latent heat of fusion at near room temperature. When the system of U.S. Pat. No. 4,367,788 is used to insulate a building, heat that penetrates the insulation system is stored in the phase-change material by its conversion from solid phase to liquid phase, and the heat is then released when the temperature falls as the phase-change material converts from liquid to solid phase, thus reducing the heat transferred between the exterior to the interior of the building through the action of the phase-change material and its latent heat of fusion.
Cryogenic cooling systems for space-based sensors have also been the subject of development. For example, U.S. Pat. No. 4,671,080 discloses a closed cryogenic cooling system using an electrochemical pump to provide a pressurized gas stream of hydrogen (or oxygen) in a high pressure flow to one or more heat exchangers to cool the hydrogen gas stream below its inversion temperature, followed by an expansion through a Joule-Thomson flow restrictor into a load heat exchanger to provide cryogenic cooling for the load. In space craft applications, it is disclosed that the Joule-Thomson flow restrictor can be a metal, foam or porous sinter and that the conversion of hydrogen between its para and ortho states should be considered in the design of such systems with the incorporation of suitable catalysts in the high pressure and low pressure sides of a regenerative heat exchanger between the Joule-Thomson valve and the electrochemical pump.
U.S. Pat. No. 4,393,039 discloses the use of the vapor from a solid hydrogen mass (maintained below its triple point by a cryogenic dewar) and the endothermic catalytic conversion of the hydrogen vapor from its para state to ortho state in cooling a load.
Notwithstanding such development, a need remains for lightweight, passive systems to provide effective thermal insulation and protection of the mechanical and electrical systems and structures of flight vehicles, particularly those flight vehicles exposed to extraordinary conditions of atmosphere and temperature, such as the X-30 hypersonic aircraft and other such space flight vehicles.